LM49200, LM49200TLEVAL

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SNAS459A – MAY 2009 – REVISED APRIL 2013

A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides

differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output

power is possible as compared to a single-ended amplifier under the same conditions. This increase in attainable

output power assumes that the amplifier is not current limited or clipped.

A bridge configuration, such as the one used in LM49200, also creates a second advantage over single-ended

amplifiers. Since the differential outputs are biased at half-supply, no net DC voltage exists across the load. This

eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier

configuration. Without an output coupling capacitor, the half-supply bias across the load would result in both

increased internal IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an

increase in internal power dissipation. The power dissipation of the LM49200 varies with the mode selected. The

maximum power dissipation occurs in modes where all inputs and outputs are active (Modes 6, 7, 8, 9, 10, 11,

13, 14, 15). The power dissipation is dominated by the Class AB amplifier. The maximum power dissipation for a

given application can be derived from the power dissipation graphs or from Equation 1.

PDMAX = 4*(VDD)

2 / (2π2R

L)

(1)

It is critical that the maximum junction temperature (TJMAX) of 150°C is not exceeded. TJMAX can be determined

from the power derating curves by using PDMAX and the PC board foil area. By adding additional copper foil, the

thermal resistance of the application can be reduced from the free air value, resulting in higher PDMAX. Additional

copper foil can be added to any of the leads connected to the LM49200. It is especially effective when connected

to VDD, GND, and the output pins. Refer to the application information on the LM49200 reference design board

for an example of good heat sinking. If TJMAX still exceeds 150°C, then additional changes must be made. These

changes can include reduced supply voltage, higher load impedance, or reduced ambient temperature. Internal

power dissipation is a function of output power. Refer to the curves in Typical Performance Characteristics for

power dissipation information for different output powers and output loading.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply

rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as

possible. Typical applications employ a 5V regulator with 10µF tantalum or electrolytic capacitor and a ceramic

bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of

the LM49200. The selection of a bypass capacitor, especially CB, is dependent upon PSRR requirements, click

and pop performance, system cost, and size constraints.

GROUND REFERENCED HEADPHONE AMPLIFIER

The LM49200 features a low noise inverting charge pump that generates an internal negative supply voltage.

This allows the headphone outputs to be biased about GND instead of a nominal DC voltage, like traditional

headphone amplifiers. Because there is no DC component, the large DC blocking capacitors (typically 220

μF)

are not necessary. The coupling capacitors are replaced by two small ceramic charge pump capacitors, saving

board space and cost. Eliminating the output coupling capacitors also improves low frequency response. In

traditional headphone amplifiers, the headphone impedance and the output capacitor from a high-pass filter that

not only blocks the DC component of the output, but also attenuates low frequencies, impacting the bass

response. Because the LM49200 does not require the output coupling capacitors, the low frequency response of

the device is not degraded by external components. In addition to eliminating the output coupling capacitors, the

ground referenced output nearly doubles the available dynamic range of the LM49200 headphone amplifiers

when compared to a traditional headphone amplifier operating from the same supply voltage.

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